[time-nuts] Pitfalls of Digital 'Scopes
jfor at quik.com
Mon Jun 6 17:40:05 UTC 2011
As some of you know, I've been working on restoring a WW II LORAN-A
system. To test it, I've been using a Tektronix TDS1002 Digital 'Scope and
a Military TS-251 Test Set.
First a bit about the LORAN-A signal. It is a precisely timed Master pulse
of roughly 2 MHz RF. The spacing of the Master Pulse is precisely crystal
controlled, but the RF is just gated... the carrier is not locked to the
Master Pulse. It looks sort of like this:
| Master Pulse start.
Anyway, I was triggering on the Master Pulse and looking at several
succeeding ones, all generated by a TS-251 Test Set.
For the longest time, I could not figure out why the successive Master
Pulses envelopes were bouncing around all over the place by perhaps a
factor of 5.
Eventually, I worked out that most of the samples taken by the digital
scope were being taken during the quiet time, and the actual LORAN pulse
was being badly undersampled. This resulted in displayed waveform that had
almost nothing to do with what the system was really doing. Also, because
of slight trigger uncertainties, the LORAN pulse was being "smeared" in
IMO, the lesson is that digital scopes do not always accurately depict
what a circuit is doing. Even a $50 analog 'scope would never have this
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